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Preliminary: English is not my native language.

In the Rulebook for Apocalypse Battles two examples are listed under the heading "Buildings".
Example 2 speaks of a terrain piece and that an enemy unit must be visible from one point of the terrain and that the distance between the target and closing unit is measured from the nearest point on the occupied ground.
This means that it is irrelevant whether a model of the shooting unit sees the goal, the "building must see the target".

So, if I set up a unit on the ground floor of a ruin (where there are no windows), I can still shoot at units within range of the ruin, and these units can also be fired upon, because only the building and not the unit will be fired at.
Thus, units that possess barrage fire (do not need line of sight) can not hide in a ruin but can still be fired at?
You could hide behind walls, etc. at most?
Am I correct?

Yes, correct, but you could hide behind the building, provided its walls block LOS.

Once a unit is occupying terrain the terrain piece is the unit for all aiming purposes. You trace LoS to the terrain when you want to shoot the unit. You measure from the terrain when you are shooting with the unit.

Thx a lot!